jon lapook is our medical correspondent. john, seth just said the c.d.c. is investigating how the vials were contaminated. how might they have been wentaminated? haat are the possibilities? >> reporter: scott, that medication is supposed to be prepared in a sterile environment but if there's a breakdown in the system there could be contamination in the fungus. it's common in soil, decomposing leaves and even floating in the air. >> pelley: if the fungus is everywhere, as you describe it, why is it so dangerous in this case? >> reporter: in somebody with a normal immune system the lungs filter it out. ouen you inject it directly into the spine, that's like a straight highway to the brain. >> pelley: jon, thank you very ver. it has been more than three weeks since that deadly terrorist attack on the u.s. s.nsulate in benghazi, libya. today, the libyan government finally allowed a team of f.b.i. agents to see the crime scene. four americans were killed in that attack, of course, including ambassador chris stephens. the agents, with a u.s. military escort, spe